ME218A: Soccer Pinball scoreboard

Posted on Feb 7, 2014

The diagram below illustrates the high level connections required to drive a single 7-segment display on the scoreboard. The below circuit was repeated 4 times - once for each of the four 7-segment LED displays. Each circuit takes a single data line as its input (C32 outputs T4-T7), and uses a shared clock signal (C32 output T3) that is common to

ME218A: Soccer Pinball scoreboard
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all of the 4 LED display circuits. The scoreboard therefore requires 5 pins total on the C32 to operate, along with 5V power, 13. 6V power, and ground. The single input to each 7-segment LED display is not adequate to directly control the operation of the display. To control the segments independently of one another, the scoreboard uses a 74HC164 8-bit serial-in, parallel-out shift register. This particular shift register operates by performing and AND operation on the two input lines and assigning this logical value to the first output pin of the shift register each time a rising edge is detected on the clock input. The value in the first output subsequently shifts to the second output, and so on down the line. By pulsing the clock 8 times, the full set of 74HC164 outputs can be assigned. With 4 data lines and a shared clock pulse, the C32 can simultaneously control the states of 32 different outputs, and completely change their values in 8 clock pulses. The only limitation on the use of the shift registers is the requirement that the output circuitry be functionally insensitive to the transient output states which occur when the contents of the register are being changed. In the case of the LED display, these transient states are manifested as almost imperceptible flickering of individual segments during the refresh process. The choice of a common cathode LED display requires that the individual segments of the LED...

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